Sluggish cognitive tempo (SCT) is an a subtype of the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders'attention-deficit/hyperactivity disorder, Predominantly Inattentive Type (ADHD/IA). In many cases of ADHD/IA, diagnosed patients have subclinical levels of hyperactivity-impulsiveness. Hyperactive symptoms may also be outgrown while inattentive symptoms remain into adulthood. In contrast, SCT has inattentive features from the start without any history of hyperactivity-impulsiveness. Some researchers believe this is a different disorder from ADHD with a different form of inattentivity.

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People with ADHD Combined Type (ADHD/C) and ADHD/IA with subthreshold hyperactive-impulsive symptoms (ADHD/C for both hereon) show a qualitatively different kind of attention deficit from those who have no major history of hyperactivity-impulsivity. SCT children show lower selective attention and sustained focus than controls. Instead of being unable to select and filter sensory input as is the case in SCT, people with ADHD/C have problems with executive function, including working memory, which leads to a different set of inattentive symptoms (distractibility, low goal persistence). People with SCT are more disorganized and likely to make careless mistakes than simply become distracted and not come back to a required task.

In contrast to other types of ADHD, people with a sluggish cognitive tempo are characteristically hypoactive and reserved; they are more likely to be confused and daydreaming than aggressive and out of control. Comorbid psychiatric disorders are of the internalizing variety: anxiety, depression, and social withdrawal. People with the other types of ADHD are more likely to externalize: substance abuse, oppositional-defiant disorder, conduct disorder, and even antisocial personality disorder (i.e., psychopathy).